Document current expectations for curl commands

What does this MR do?

As part of researching long options vs short options for Git in Verbose git command line arguments in empty rep... (!122213 - merged), I realized:

• We have already stated similar preferences for curl commands.
• We run an error-level test for it in scripts/lint-doc.sh in lines 13-23.
• Until Document non-Vale/markdownlint tests in lint-do... (!122359 - merged) landed, the existence of those tests in scripts/lint-doc.sh were not acknowledged anywhere in our docs
• We've now documented the existence of the tests but have nothing in our style guide about our preference for long flags in curl commands.

In addition to acknowledging the existence of the long-flag rule we already enforce, I'm adding one more line that I think is common sense: an explanation of how to wrap long lines for legibility. Code blocks with long lines (looking at you, API curl commands) get left-right scrollbars, and the first part of most of the commands is the same. The parts that change are typically hidden behind the scrollbar. As I've been gradually cleaning up the ancient API pages for Source Code and Code Review, I've been adding in these line breaks.

Compare:

• the horizontal scrolling needed in https://docs.gitlab.com/ee/api/merge_requests.html#set-a-time-estimate-for-a-merge-request to see the useful part of the example, versus
• the immediate legibility of https://docs.gitlab.com/ee/api/protected_branches.html#example-create-a-push_access_level-record

Routing over to Suzanne because this is a style guide thing, but nodding in @eread and @marcel.amirault's direction. They both did significant review of !122359 (merged) and will be unsurprised to see this merge request as a follow-up.

Related issues

• Related to Document non-Vale/markdownlint tests in lint-do... (!122359 - merged) where the existence of the test was documented
• Related to Verbose git command line arguments in empty rep... (!122213 - merged) where I was researching long vs short options for Git, and realized that while we had no stance for Git commands, we were already enforcing long flags in curl commands but had never added them to the style guide

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